The present invention relates to mobile communication systems and more specifically it concerns a method for channel assignment in a system that exploits the space division multiple access (SDMA) technique.
Mobile communication systems are characterised by a continuous increase in the number of users, who have to share limited resources, particular in regard to the available frequency band. It is therefore of interest to study new access techniques which allow a more effective exploitation of the available resources. One of the techniques proposed for this purpose is the SDMA technique, which allows increasing system capacity by exploiting spatial separation between users. In other words, users who are sufficiently separated angularly with respect to the base radio station can share the same channel, i.e. use the same frequency and the same time slot, if FDMA-TDMA (Frequency Division Multiple Accessxe2x80x94Time Division Multiple Access) techniques are used.
The application of this technique is made possible by the use in the base stations of adaptive array antennas (the so-called xe2x80x9csmart antennasxe2x80x9d), that allow distinguishing between useful signals and interfering signals and computing their directions of arrival. By following the changes in the directions of arrival both of the useful and of the interfering signals, the antennas continuously adapt the radiation diagram, thereby following the terminal of interest with the main lobe of that diagram and placing its zeroes in correspondence with the interfering terminals. This yields a better carrier/interference (C/I) ratio both for the base station and for the mobile terminals. The capability of these antennas of distinguishing between useful signals and interfering ones allows obtaining a spatial orthogonality between users, which enables to re-use the same channel for multiple users, provided they are at different angular positions with respect to the base station, i.e. provided there is an angular separation exceeding a minimum threshold between the respective radiated beams. An additional requirement is that users sharing the same channel must have similar power levels, since array antennas can attenuate the interfering signals of a terminal that uses the same channel as the terminal transmitting the useful signal only to a limited extent.
The possibility of using the same channel for multiple users clearly influences channel assignment procedures both at the call set-up and in case of handover: in SDMA systems handovers become necessary both when the angular separations of users employing the same channels decreases below the threshold (intra-cell handover) and when the C/I ratio drops below a certain value. That decrease below the threshold of the angular separation between users of the same channel shall hereafter be referred to as xe2x80x9ccollisionxe2x80x9d.
The channel assignment procedure must try to meet the following requirements:
the angular separation between two terminals using the same channel shall exceed the minimum threshold;
the probability must be minimised that, for a given terminal, a new channel assignment (i.e. a handover) becomes necessary while a communication is in progress or, stated in different terms, the time between two successive channel assignments relating to the same terminal must be made as long as possible.
These requirements depend on the position of the mobile terminals, on their speed and on the topographic characteristics of the cell.
It is easy to understand that in order to meet these requirements it is convenient to assign to a terminal, whenever possible, a xe2x80x9cregularxe2x80x9d channel, i.e. one not shared by other terminals; only if no free regular channels exist, a shared channel, chosen with an appropriate criterion, will be assigned.
The document xe2x80x9cTowards a Channel Allocation Scheme for SDMA-based Mobile Communication Systemsxe2x80x9d, Institute of Computer Science, Universitxc3xa4t Wxc3xcrzburg, Research Report Series, Report No. 104, February 1995, proposes to assign the channels by taking into account solely the position (angular separation and distance) of the terminals, and it reports some performance assessments. An assignment method of this kind entails as a consequence a non-optimal utilisation of resources, since on one hand it cannot assign the same channel to users who, although angularly close, cannot collide or will possibly collide only after a very long time due to the topographic characteristics and/or to the direction and velocity of displacement, and on the other hand it can instead cause relatively frequent handovers, if the angular separation that had induced to assign a shared channel reduces rapidly due to the topographic characteristics of the cell and/or to the users"" direction and velocity of displacement.
The present invention provides instead a method of assigning channels which takes into account not only the position of the terminals, but also their characteristics of mobility as well as the topographic characteristics of the cell.
According to this method, under channel re-use conditions, the channel to be assigned to a terminal (xe2x80x9crequesting terminalxe2x80x9d) is determined by assessing an assignment cost of each channel already in use, which cost is a function of the mean time that elapses between two successive channel assignments to said terminal, and allows identifying the channel that enables to maximise that time. This assessment is performed by using cost coefficients that take into account information on the topology of the cell and on the mobility of the terminals and that are determined in a system initialisation phase through the following operations:
subdividing a cell of the system into sectors;
tracking the active terminals during their displacement inside the cell and storing the instant at which each terminal enters each sector travelled by it in the course of that displacement;
detecting collisions between two terminals, a collision being recognised as a decrease in the angular separation between said terminals below a pre-set threshold, and the instant wherein said collision occurs;
storing, upon that detection, a collision event both for each of the sector pairs wherein two terminals involved in a collision were simultaneously present in the course of their movements within the cell, and for the individual sectors travelled by the two terminals;
counting the number of collisions for the different sector pairs and for the individual sectors;
computing the mean time elapsing between the entry of a terminal into one of the sectors considered and the instant of collision;
determining a cost coefficient for each sector pair, as a function of the mean time necessary for this collision to occur.
Preferably, such cost coefficients are inversely proportional to said mean time, which may be weighted with the probability that the collision takes place. In this second case, the operations that lead to the determination of the cost coefficients shall also comprise the computation, for each sector pair, of the probability that a collision occurs between the terminal which at the instant of the channel assignment request is in the first sector of the pair and a terminal which at the same instant is in the second sector of the pair.
The angular separation threshold can be a function of the minimum acceptable C/I ratio.